Process for the production of multiple cross-linked hyaluronic acid derivatives

ABSTRACT

The present invention relates to a process for the production of cross-linked hyaluronic acid (HA) derivatives, in particular multiple, e.g. double cross-linked hyaluronic acid derivatives. The invention also provides novel cross-linked HA derivatives, products containing them and their uses in medical and pharmaceutical and cosmetic applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/920,286, filed Aug. 2, 2001, which is a continuation of InternationalApplication No. PCT/GB00/00321, having an International Filing Date ofFeb. 3, 2000, which claims the benefit of United Kingdom ApplicationSerial No. 9902412.7, filed Feb. 3, 1999.

The present invention relates to a process for the production ofhyaluronic acid (HA) derivatives, in particular multiple, eg doublecross-linked hyaluronic acid derivatives, to novel cross-linkedderivatives so obtained, to products containing them and their uses incosmetic, medical and pharmaceutical applications.

HA is a member of a class of polymers known as glycosaminoglycans. HA isa long chain linear polysaccharide and is usually present as the sodiumsalt which has a molecular formula of (C₁₄H₂₀NNaO₁₁)_(n) where n canvary according to the source, isolation procedure and method ofdetermination. However, molecular weights of up to 14×10⁶ have beenreported.

HA and its salts can be isolated from many sources including humanumbilical cord, rooster combs' and nearly all connective matrices ofvertebrate organisms. HA is also a capsular component of bacteria suchas Streptococci as was shown by Kendall et al, (1937), Biochem. Biophys.Acta, 279, 401-405; it may therefore also be obtained by fermentationmethods. For example, the present applicant's U.S. Pat. No. 5,411,874describes a method for producing hyaluronic acid by continuousfermentation of Streptococcus equi.

HA is non-immunogenic and therefore has great potential in medicine.Because of its visco-elastic properties HA having a high molecularweight (over 1 million) has been found to be particularly useful in avariety of clinical fields, including wound treatment, ophthalmicsurgery and orthopaedic surgery. HA is also potentially useful in avariety of non-medical fields, such as cosmetic applications.

However, the use of HA in certain of these applications is limited bythe fact that following administration to humans HA is readily degradedby enzymes such as hyaluronidases and by free radicals. Furthermore, HAis soluble in water at room temperature, which can also make it lesssuited to certain applications. Various attempts have therefore beenmade to prepare more stable forms of HA, in particular by cross-linkingthe HA molecules.

Thus, U.S. Pat. No. 4,582,865 (Biomatrix Inc.) describes the preparationof cross-linked gels of hyaluronic acid which are formed bycross-linking HA either by itself or mixed with other hydrophilicpolymers using divinyl sulfone as the cross-linking agent. It appearsthat in this case the cross-linking occurs via the hydroxyl groups ofHA.

U.S. Pat. No. 5,550,187 (Collagen Corporation) describes a method forpreparing cross-linked biomaterial compositions which involves mixing abiocompatible polymer, which is preferably collagen but may be selectedfrom other polymers including hyaluronic acid, with a sterile drycross-linking agent such as a synthetic hydrophilic polymer.

U.S. Pat. No. 5,578,661 (Nepera Inc.) describes a gel forming system foruse as a wound dressing which is formed from three main components, thefirst being a water soluble polymer, the second being an acid-containingpolymer and the third being a polysaccharide or amino-containing polymersuch as hyaluronic acid. In this case the cross-linking appears to bevia ion-bonding.

U.S. Pat. No. 5,644,049 (Italian Ministry for Universities andScientific and Technology Research) describes a biomaterial comprisingan inter-penetrating polymer network (IPN) wherein one of the polymercomponents is an acidic polysaccharide such as hyaluronic acid and thesecond polymer component may be a synthetic chemical polymer. The twocomponents may be (but are not necessarily) cross-linked.

Tomihata and Ikada have reported cross-linking of HA using a watersoluble carbodiimide as cross-linking agent. It was postulated thatcross-linking took place via ester groups. The cross-linking reactionwas also carried out in the presence of L-lysine methyl ester, whichappeared to give additional cross-linking via amide bonds to the lysineester. (J. Biomed. Mater. Res., 37, 243-251, 1997).

U.S. Pat. No. 5,800,541 describes collagen-synthetic polymer matricesprepared using a multiple step reaction. The first step involvesreacting collagen with a synthetic hydrophilic polymer; the resultingmatrix may then be modified in a second reaction step which may involvecross-linking or conjugating the matrix with a synthetic polymer,coupling biologically active molecules or glycosaminoglycans to thematrix, cross-linking the matrix using conventional chemicalcross-linking agents or modifying the collagen in the matrix by means ofchemical reaction. In this process, the initial collagen-syntheticpolymer matrix appears to be cross-linked via only one type of bond, andthe additional process steps serve to introduce further chemicalsubstances which may form different types of bonds. However, it does notappear that any two of the substances forming the product will be linkedto each other by more than one type of bond.

International patent application WO 97/04012 (Agerup) describespolysaccharide (which may be inter alia hyaluronic acid) gelcompositions which are prepared by forming an aqueous solution of thepoylsaccharide, initiating cross-linking in the presence of apolyfunctional cross-linking agent, sterically hindering thecross-linking reaction from being terminated before gelation occurs (egby diluting the solution) and then reintroducing sterically unhinderedconditions (eg by evaporating the solution) so as to continue thecross-linking to a viscoelastic gel. There is no suggestion in thisapplication that different types of bonds are formed in the twocross-linking stages.

None of the aforementioned documents describe products in whichmolecules of HA are linked to each other by means of two different typesof cross-linking bonds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a solid state ¹³C NMR spectrum of hyaluronicacid.

FIG. 2 is a graph showing a solid state ¹³C NMR spectrum of a singlycross linked film of hyaluronic acid that was cut into fine mesh for NMRanalysis.

FIG. 3 is a graph showing a solid state ¹³C NMR spectrum of a doublecross linked hyaluronic acid gel that was milled to a fine powder forNMR analysis.

FIG. 4 is a graph showing a solid state ¹³C NMR spectrum of the doublecross linked hyaluronic acid powder shown in FIG. 3 in combination withan internal standard, tetrakis(trimethyl)silane, which has a chemicalshift of 3.2 ppm.

We have now found that hyaluronic acid may be cross-linked by twodifferent types of cross-linking bonds, to effect a ‘doublecross-linking’. The formation of different types of bonds is achieved byeffecting the cross-linking via different functional groups. The bondsso formed can therefore be described as functional bonds. Thus forexample one type of bond may be formed by cross-linking via hydroxylgroups and a different functional bond formed by cross-linking via e.g.carboxyl groups. Such multiple cross-linking has been found to result ina high degree of cross-linking with improved biostability of HA.

In a first aspect therefore, the present invention provides a processfor the preparation of multiple cross-linked derivatives of hyaluronicacid, which process comprises cross-linking HA via two or more differentfunctional groups.

The crosslinking of each type of functional group may be effected bycontacting HA with one or more cross-linking agents, simultaneously orsequentially, as described in more detail hereinbelow.

In this specification, ‘multiple crosslinked HA’ refers to a hyaluronicacid derivative wherein a molecule of HA is cross-linked to anothermolecule of HA by means of two or more different types of functionalbond. Similarly, ‘double crosslinked HA’ refers to a hyaluronic acidderivative wherein a molecule of HA is cross-linked to another moleculeof HA by means of two different types of functional bond and ‘singlecrosslinked HA’ refers to a hyaluronic acid derivative wherein amolecule of HA is cross-linked to another molecule of HA by means ofonly one type of functional bond

The functional groups which are mainly responsible for cross-linking ofHA molecules are the hydroxyl and carboxyl groups. Hydroxyl groups maybe cross-linked via an ether linkage and carboxyl groups via an esterlinkage. If desired the HA may be chemically modified prior tocross-linking to form other chemically reactive groups. Thus for exampleHA may be treated with acid or base such that it will undergo at leastpartial deacetylation, resulting in the presence of free amino groups.Said amino groups may be cross-linked via an amide (—C(O)—NH—); imino(—N═CH—) or amine (—NH—CH—) bond. An imino bond is a precursor of anamine bond and an imino linkage can be converted into an amine linkagein the presence of a reducing agent.

Cross-linking agents which may be used in the process of the presentinvention include those well-known in the art, for example formaldehyde,glutaraldehyde, divinyl sulfone, a polyanhydride, a polyaldehyde, apolyhydric alcohol, carbodiimide, epichlorohydrin, ethylene glycoldiglycidylether, butanediol diglycidylether, polyglycerolpolyglycidylether, polyethylene glycol, polypropylene glycoldiglycidylether, or a bis- or poly-epoxy cross-linker such as1,2,3,4-diepoxybutane or 1,2,7,8-diepoxyoctane.

To form an ether linkage the cross-linking agent is preferably selectedfrom formaldehyde, gluteraldehyde, divinyl sulfone and, in alkalineconditions, bis and poly epoxides. Preferably the crosslinker contains ahydrophobic hydrocarbon segment, e.g. 1,2,3,4,-diepoxybutane, or mostpreferably 1,2,7,8-diepoxyoctane.

To form an ester linkage the cross-linking agent is preferably selectedfrom polyhydric alcohols, carbodi-imides, polyanhydrides, carboxylicacid chlorides and, in acid conditions, bis and poly epoxides.Preferably the crosslinker contains a hydrophobic hydrocarbon segment,e.g. 1,2,3,4,-diepoxybutane, or most preferably 1,2,7,8-diepoxyoctane.

An amide linkage is preferably formed using a cross-linking agentselected from carbodi-imides in the presence of amines, carboxylic acidanhydrides and chlorides (with de-acetylated HA), and diisocyanates.

An amine linkage is preferably formed using a cross-linking agentselected from an epoxide, or glutaraldehyde with a reducing agent, inthe presence of amino groups in deacylated HA.

An imino linkage (schiff base bond) may be formed using glutaraldehydein the presence of amino groups in deacylated HA.

A sulfone linkage is preferably formed using a sulfonyl chloride.

In one embodiment of the present invention, the different functionalbonds may be formed sequentially, in a multi-step process, which may beachieved either by using a different cross-linking agent for each stageor by using the same cross-linking agent at each stage and adjusting thereaction conditions to control the specific cross-linking reactionrequired.

Thus, to achieve multiple, e.g. double, cross-linking in a step-wisemanner according to the present invention a first cross-linking reactionis carried out, for example using one of the methods described below.When this is complete, or has progressed as far as required, a furthercross-linking agent is added to the reaction mixture to effect thesecond cross-link. The further cross-linking agent may be the same ordifferent from the first. When a different cross-linking agent isemployed it will generally be selected such that without changing thereaction conditions, it will form a different type of functional bond.However, when the same cross-linking agent is employed to form bothcross-links, the reaction conditions should be adjusted accordingly inorder to form a different type of bond. Those skilled in the art willreadily be able to select an appropriate cross-linking agent and theappropriate reaction conditions to form the desired bond.

For the avoidance of doubt, it is noted that if the same cross-linkingagent is used under the same reaction conditions at each step, this willresult in only one type of linkage, i.e. it will give a singlecross-linked product, albeit produced in two or more stages.

It will be appreciated that when the two or more functional bondsaccording to the invention are formed sequentially, i.e. in amulti-stage reaction, the cross-link formed in the first stage of thereaction should be sufficiently strong to withstand the reactionconditions needed to form the second or subsequent cross-link(s). Thus,the stronger of the two (or more) bonds should be formed first. Thiswill be readily apparent to the skilled worker and if necessary can bedetermined by means of routine experimentation.

Thus, when the cross-links are to be formed via hydroxyl and carboxylgroups it will be recognised that the first-stage cross-linking shouldbe effected via the hydroxyl groups to give an ether linkage and thesecond-stage cross-linking will then be effected via the carboxylgroups, to give an ester link.

An ether bond may be formed using an epoxide crosslinker under alkalineconditions, preferably at a pH of 10 or more or, providing the HAcontains no free amino groups, using glutaraldehyde as the crosslinkingagent under acid conditions e.g. pH4 or less. An ester bond may beformed with an epoxide crosslinker under acid conditions e.g. pH4 orless.

Thus, for example, a first cross-linking reaction to form an etherlinkage may be carried out using an epoxide such as 1,27,8-diepoxyoctane under alkaline conditions, preferably at a pH of 10 ormore, for example in the range of pH 10 to pH12. A second cross-linkingreaction to form an ester linkage may subsequently be effected employingthe same cross-linking agent, and adjusting the pH of the reactionmedium to pH4 or less, for example in the range pH 4 to pH2.Alternatively different cross-linking agents may be used in each step,in which case it may not be necessary to adjust the reaction conditions.Thus for example a first cross-linking reaction may be carried out usingglutaraldehyde under acidic conditions to form an ether link, followedby reaction with an epoxide cross-linker also under acid conditions toform an ester link.

The ratio of cross-linking agent to HA employed at each stage of thisprocess will generally be in the range 1:10 to 10:1 by weight.

The individual cross-linking reactions may be carried out according tomethods known generally in the art.

Thus, the HA utilised as the starting material may be in the form of afilm or in solution.

When HA film is employed, this may be suspended in a suitable solventtogether with a cross-linking agent. The reaction medium preferablycomprises an organic solvent such as acetone, chloroform, or an alcohole.g. ethanol or isopropanol, admixed with an aqueous acidic or alkalinesolution. An acidic solution preferably has a pH of 4 or less and analkaline solution preferably has a pH of 10 or above. The cross-linkingreaction suitably takes place at a temperature in the range of 15 to 30°C. e.g. ambient temperature.

Preferably, when HA film is employed as starting material an ethercross-link is first formed with either an epoxide under alkalineconditions or, providing there are no free amino groups present,glutaraldehyde under acid conditions, followed by formation of an estercross-link using epoxide under acid conditions. If the HA has beendeacetylated to provide free amino groups, a schiff base with an iminolinkage can be formed by reacting with glutaraldehyde under acidicconditions. An imino bond can be converted to an amine bond using areducing agent.

HA may also be employed as an aqueous acidic or alkaline solution towhich the cross-linker is added. Under acidic conditions the pH of thestarting solution is preferably pH4 or lower and for an alkalinesolution the pH is preferably pH10 or above. The concentration of HA issuitably in the range 1 to 10% w/w. The reaction may be effected at atemperature in the range of 15 to 50° C. The time for completion of thecross-linking reaction may in general vary from about an hour to a fewdays.

Preferably, when an HA solution is employed an ether cross-link is firstformed with an epoxide under alkaline conditions, followed by formationof an ester cross-link using an epoxide (preferably the same epoxide asin the first step) under acidic conditions.

Alternatively, HA solution may be subjected to a first cross-linkingreaction, the intermediate product dried to form a film and said filmsubjected to a further cross-linking reaction as described above to givea double cross-linked product in the form of a film. Preferably, toobtain a double cross-linked HA according to this procedure, an ethercross-link is first formed with an epoxide under alkaline conditions,followed by formation of an ester cross-link using an epoxide(preferably the same epoxide as in the first step) under acidicconditions.

In another embodiment of this invention, multiple cross-linking of HA,in particular double cross-linking, may be effected in a single stepreaction, by contacting HA simultaneously with two differentcross-linking agents, suitable for cross-linking two differentfunctional groups under the same conditions. Thus, for example, to formboth ether and ester groups in a single step HA may be contacted with amixture of glutaraldehyde and 1,2,7,8-diepoxyoctane.

The ratio of cross-linking agent to HA employed at each stage of thisprocess will generally be in the range 1:10 to 10:1 by weight.

The precise nature of the product may be varied by appropriate selectionof reaction conditions so as to control the degree of cross-linking andhence the properties of the product. Factors which influence the degreeof crosslinking and hence the nature of the final product include theform of the HA starting material employed, the feeding ratio ofcrosslinking agent to HA, the reaction time, temperature and the pH. Theproduct may be obtained in the form of a gel or film and may be clear oropaque. The water absorption capacity and biostability will varydepending on the precise nature of the product.

A product according to the invention may be obtained in the form of afilm or sheet by employing HA starting material in the form of asolution, film or sheet and carrying out the process without stirring.It will be appreciated that when HA is employed in the form of a film orsheet, this will absorb water when placed in aqueous solution such asPBS buffer and swell to form a gel. If desired an intermediate film mayoptionally be formed after the first cross-linking step, as describedabove. The product may be clear or opaque, depending upon the degree ofcross-linking which occurs. Highly cross-linked HA products aregenerally opaque and may even be white in color.

A product according to the invention in the form of a gel may beobtained by hydration of a film, which may for example be prepared asdescribed above. If necessary the film may be subdivided into smallpieces to facilitate absorption of water.

To obtain a product according to the invention in the form of an opaquegel, the HA starting material may be employed in the form of a solution,film or sheet and the entire process effected with stirring and withoutforming a film at any stage.

Whichever cross-linking method is used, the completion of the reactioncan be routinely controlled by methods well known in the art, forexample, the reaction may be terminated by neutralizing the reactionmixture and solvent precipitation to obtain a product with the desireddegree of cross-linking.

The final product may be isolated from the reaction medium byconventional procedures.

In a preferred embodiment the present invention provides a process forpreparing double cross-linked HA, said process comprising contacting HAwith one or more cross-linking agents under conditions suitable forforming two different bonds between the HA molecules. Preferably thecross-linking reactions are effected sequentially. Thus, the two-stageprocess according to the invention comprises:

-   -   (a) cross-linking HA via a first functional group and        subsequently    -   (b) further cross-linking the product of (a) via a second        functional group, wherein said first and second functional        groups represent different chemical entities.

It will be understood that when a product containing more than twodifferent cross-links is required, this may be prepared by anappropriate combination of sequential or simultaneous cross-linkingreactions as described above.

Cross-linked HA prepared according to the present invention contains atleast two different types of cross-linking bonds, for example both etherand ester bonds.

It is believed that multiple (e.g. double) cross-linked HA derivativesprepared according to the present invention are themselves novel. Thus,in a further aspect the present invention provides multiple cross-linkedHA (i.e. HA cross-linked via two or more different functional bonds)obtainable by the process described hereinbefore. Preferably theinvention provides double cross-linked HA obtainable by the processdescribed hereinbefore.

In a further aspect the present invention provides HA cross-linked toitself (i.e. to a further molecule of HA) wherein the HA is crosslinkedby at least two different types of bond. Preferably the HA is doublecross-linked HA.

Double-crosslinked HA according to the present invention may have adegree of cross-linking in the range 10 to 50%, eg 15 to 30, preferably20 to 25% (where 100% is represented by cross-linking of all OH groupsat the C6 position and all COOH groups at the C5 position). The degreeof cross-linking may be measured by elemental analysis or solid stateNMR analysis.

The ratios of the different functional bonds in the product will varydepending on the types of functional bonds present and the reactionconditions used to form them. For a double cross-linked productcontaining ether and ester bonds the ratio of these bonds may vary from50:50 to 95:5, eg 60:40 to 80:20 ether:ester bonds.

In general a product according to the present invention has a greaterdegree of cross-linking, that is to say, a denser network of cross-linksthan does single cross-linked HA. A higher degree of cross-linking hasbeen found to reduce the water absorption capacity of the cross-linkedHA, resulting in greater stability in aqueous solution. In additiondouble cross-linked HA has been found to exhibit greater stabilityagainst degradation by hyaluronidase, and against degradation due tofree radicals, indicating an increased biostability.

An opaque product according to the present invention generally has ahigher degree of cross-linking and hence lower water absorption capacityand greater stability, than a clear product. Such products are suitablefor long term implantation.

A clear product e.g. a clear film according to the present invention hashigher water absorption capacity than an opaque product and suchproducts are particularly suitable for dermal implants, wound healing(absorption of exudate) and resorbable short-term implantation.

The multi-step process described above is preferred when a highlycross-linked product with low water absorption capacity is desired.Simultaneous cross-linking generally results in a water-insolubleproduct, but with higher water absorption capacity than a productprepared using a multi-stage (e.g. two-step) process under similarconditions.

Furthermore it has been found that using a first crosslinked HA film forthe second cross-linking step provides a product (which may be in filmform or may be converted into a gel) with lower water absorptioncapacity than double cross-linked HA prepared from HA solution undersimilar crosslinking conditions (ie with no intermediate filmformation). Indeed it has been found that the water absorption capacityof the resulting products can vary from 400% to 1000% for film and gelstarting materials respectively.

Cross-linked HA derivatives according to the present invention may beused in a variety of pharmaceutical, medical (including surgical) andcosmetic applications.

Thus, they may for example be useful in promoting wound healing, e.g.,as a dermal wound dressing.

They may also be useful in preventing adhesion e.g. preventing tissuegrowth between organs following surgery.

Crosslinked HA derivatives according to the present invention may alsofind application in the ophthalmic field e.g. for vitreous fluidreplacement, as corneal shields for delivery of drugs to the eye or aslenticules.

Crosslinked HA derivatives according to the present invention may alsobe useful in surgery, for example as solid implants for hard tissueaugmentation e.g. repair or replacement of cartilage or bone, or forsoft tissue augmentation, as breast implants, or as coating for implantsintended for long term use in the body, such as breast implants,catheters, cannulas, bone prostheses, cartilage replacements, mini pumpsand other drug delivery devices, artificial organs and blood vessels,meshes for tissue reinforcement, etc. They may also be used as jointlubricants in the treatment of arthritis.

A further use for the derivatives of the present invention is in thedelivery of therapeutically active agents including in any of theaforementioned applications. Therapeutically active agents may bechemotherapeutic agents or biologically active factors (e.g. cytokines)and include anti-inflammatory agents, antibiotics, analgesics,anesthetics, wound healing promoters, cystostatic agents,immunostimulants, immunosuppressants and antiviral

The therapeutically active factors may be bound to the crosslinked HAderivative by methods well known in the art.

The crosslinked HA derivatives may be used in a variety of formsincluding membranes, beads, sponges, tubes, sheets and formed implants.

The invention will now be further illustrated by the followingnon-limiting examples.

The following procedures were used to measure stability of the products.

Methodology

Water Absorption Capacity Assessment

20 mg (Wd) of each dried cross-linked samples were immersed in PBSformulation buffer solution for 24 hours to obtain a fully swollen gel.The wet gel was filtered off and the residual water at the surface wasremoved using tissue paper. The wet gel was weighed to get Ws. Thus thewater absorption capacity (WAC) (%) can be calculated according to thefollowing formula:WAC(%)=(Ws−Wd)/Wd×100Resistance to Hyaluronidase Digestion

20 mg crosslinked HA was suspended in 6 ml PBS buffer solution (pH=7.2)containing 1000 U hyaluronidase and incubated at 37 degree C. for 24hours.

After that, the film was removed and rinsed using PBS buffer and all therinsing solution was collected to obtain total 10 ml solution. Thissolution was boiled for 30 minutes to get hyaluronidase precipitation.The solution then was centrifuged at 4000 rpm/10 minutes. Thesupernatant solution was made up to 25 ml using PBS solution in avolumetric flask. The HA concentration was measured using Carbazoleassay.

The HA weight loss due to hyaluronidase digestion can be calculatedusing the following formula:HA weight loss(%)=[HA]×25/[HA]o×100in which, [HA] is the concentration of HA, [HA]o is the original HAcontent (mg).Resistance to Free Radicals

Ferton agents are used to create free radicals, which are formed by 25microliter 0.1 ascorbic acid and 0.25 microliter 0.1M H₂O₂ in 5 ml PBSsolution. 20 mg dried sample was added to this solution for digestion.The digestion time is 24 hours at 37° C. After this, the film wasremoved and rinsed using PBS buffer and all the rinsing solution wascollected and made up to 25 ml using PBS buffer in a volumetric flask.The HA concentration was measured using Carbazole assay. The HA weightloss can be calculated using the same formula as hyaluronidasedigestion.

EXAMPLE 1

Formation of Double Crosslinked HA Film, Starting from HA Film

5 ml of HA (1%) was cast for 4 days at room temperature to get HA film.The resulting film was suspended in a mixture of CHCl₃ solvent/acidic oralkaline solution/1,2,7,8-diepoxyoctane or glutaraldehyde cross-linker.The cross-linking reaction was effected at room temperature for a fixedtime (24 hr). A further amount of cross-linking agent was added, and ifnecessary the pH adjusted, and the mixture was allowed to stand at roomtemperature for a further 24 hours, to effect the second cross-linkingreaction. The detailed cross-linking conditions are shown in Table 1.After the cross-linking, the samples were washed with IPA and acetonefor three times, immersed into IPA/deionised water (60/40) overnight andthen washed with acetone and dried in a 37° C. oven to get a constantweight.

TABLE 1 Formation of Cross-linked HA (CHA) from HA Film First SecondWater crosslinker crosslinker absorption feeding Feeding TimeTemperature capacity Name ratio* Name ratio* (hour) (° C.) pH (%) CHA-2G  2.5/1 E 0.75/1 24 h/24 h RT H+ 414.4 CHA-8 E 0.75/1 E 0.75/1 24 h/24h RT OH−/H+ 403.0 CHA-3 G  2.5/1 E 0.75/1 24 h/24 h RT H+/OH− 4430.0CHA-5 E 0.75/1 G  2.5/1 24 h/24 h RT H+ 1017.2 CHA-9 E 0.75/1 G  2.5/124 h/24 h RT OH+/H+ 4400.0 Reference Examples CHA-1 G  2.5/1 G  2.5/1 24h/24 h RT H+ 11132.5 CHA-4 E 0.75/1 E 0.75/1 24 h/24 h RT H+ 781.9 CHA-6E 0.75/1 E 0.75/1 24 h/24 h RT H+/OH− Dissolved CHA-7 E 0.75/1 E 0.75/124 h/24 h RT OH− 11989.1 *Feeding ratio: the weight ratio of HA tocrosslinker E: = 1,2,7,8-diepoxyoctane; G: = Glutaraldehyde H+represents a pH of about 4; OH− represents a pH of about 10 CHA-1,CHA-4, and CHA-7, were each prepared using the same conditions for eachcrosslinking step, giving only ether bonds (single cross-linking).

EXAMPLE 2

Formation of Double Crosslinked HA Gel from HA Solution

0.1 g of HA was dissolved in 0.25N NaOH solution or 0.25N HCl solutionto obtain HA solutions at 10% or 2.5% concentration. Cross-linking agentwas added and the mixture subjected to mechanical stirring. The firstcross-linking reaction was effected at 40° C. for a period of about 2hours. A second cross-linking reaction was effected using a furtheramount of the same cross-linker, with adjustment of the reactionconditions. Detailed reaction conditions are given in Table 2. Aftercross-linking, the formed gel was washed with IPA, acetone and extractedwith IPA/water overnight and then washed with IPA and acetonerespectively for three times. The samples were dried in a 37° C. oven toachieve a constant weight. The product was obtained as an opaque gel.

TABLE 2 Formation of Cross-linked HA (CHA) from HA Solution First Secondcrosslinker crosslinker Water feeding feeding Time absorption Name ratioName ratio (hr) Temp (° C.) pH capacity (%) CHA-11 E 1/1 E 1/1 2 h/2 h40 OH−/H+ 390.0 CHA-10 E 1/1 E 1/1 2 h/2 h 40 OH−/OH− 620.0 CHA-12 E 1/1E 1/1 2 h/2 h 40 H+/OH− 1830.0 CHA-13 E 1/1 E 1/1 2 h/2 h 40 H+/H+dissolved E: 1,2,7,8-diepoxyoctane H+ represents a pH of about 4; OH−represents a pH of about 10

EXAMPLE 3

Formation of Double Cross-linked HA (CHA) from HA Solution Via HA Film

0.1 g of HA was dissolved in 0.25N NaOH solution or 0.25N HCl solutionto obtain HA solutions at 10% or 2.5% concentration. Cross-linking agentwas added. The reaction was carried out in a Petri dish with little orno mechanical stirring. The first cross-linking reaction was effected atroom temperature for a period of about 48 or about 72 hours. Theintermediate product was dried to yield a film or sheet (depending uponthe thickness). A second cross-linking reaction was effected using themethodology described in Example 1. Detailed reaction conditions aregiven in Table 3 below. After cross-linking, the product was washed (×3)with IPA and acetone and extracted with IPA/water overnight and thenwashed with acetone. The samples were dried in a 37° C. oven to achievea constant weight and the product obtained in the form of a film orsheet.

TABLE 3 Formation of Cross-linked HA (CHA) from HA solution via HA filmFirst Second crosslinker crosslinker Water Feeding Feeding TimeTemperaure absorption Name ratio Name ratio (hour) (° C.) pH capacity(%) CHA-17 E 0.375/1 E 0.5/1 72/24 RT OH−/H+ 403.2 CHA-19 E-1 0.375/1 E0.5/1 72/24 RT OH−/H+ 1030.0 CHA-14 E 0.375/1 / / 72 RT Neutral 2419.1CHA-15 E 0.375/1 / / 72 RT H+ 2128.3 CHA-16 E 0.375/1 / / 72 RT OH−1318.6 CHA-18 E-1 0.375/1 / / 72 RT OH− 2600.4 E: 1,2,7,8-diepoxyoctane;E-1: epichlorhydrin H+ represents a pH of about 4 OH− represents a pH ofabout 10

TABLE 4 Biostability of crosslinked HA against hyaluronidase and freeradicals WEIGHT LOSS (%) Ferton digestion NO Hyaluronidase digestion(free radical) CHA-16 10.45 ± 0.21 7.89 ± 1.92 CHA-17  1.45 ± 0.92 5.63± 2.73

EXAMPLE 4

0.1 gm HA were dissolved in 2 ml 1N NaOH solution overnight to get 5% HAalkaline solution. To this solution was added 0.2 ml1,2,7,8-diepoxyoctane. 0.2 ml chloroform was then added whilst stirringat 40° C. for 30 minutes. After forming the ether cross-linkage, 2.2 ml1N HCl was added to change the pH of the solution to between 3-4. Afurther 0.2 ml 1,2,7,8-diepoxyoctane was added and 0.2 ml chloroform wasthen added whilst stirring at 40° C. for 30 minutes. After the estercross-linkage, the formed gel was precipitated with 20 ml acetone andpurified according to the same procedure as detailed in Example 2.

EXAMPLE 5

To 5 ml HA/NaOH (1N) solution, 0.5 ml epichlorhydrin and 0.2 mlchloroform were added and mixed at room temperature for 10 minutes. Thesolution was cast in a petri dish and allowed to dry as a film ofcross-linked HA (CHA-18). After neutralisation with 1N HCl, the CHA-18sample was suspended in 20 ml chloroform/0.1N acidic aqueous solution(3/1 v/v) and 0.2 ml 1,2,7,8-diepoxyoctane was added and allowed toreact at room temperature for 24 hours. The resulting sample, CHA-19,was purified according to the same procedure detailed in Example 1.

EXAMPLE 6

20 ml of 2.5% HA/NaOH (1.0N) solution was mixed with varied volumes of1,2,7,8-diepoxyoctane for 5 minutes under stirring. The mixed solutionwas then spread on to a 7 cm dimension of polystyrene non-collagencoated Petri dish with a cover. After 24 hours at room temperature, thecover was removed and the cross-linked gel was dried off for 48 hours.The dried film with controllable thickness was neutralized withacetone/HCl solution and purified with acetone/H₂O, acetone and IPA.Then the first cross-linked sheet-like material was put into anacetone/HCl solution at pH 5 and 0.4 ml 1,2,7,8-diepoxyoctane foranother 24 hours cross-linking at room temperature. The obtained sheetwas purified with acetone/water, acetone, and IPA/water, IPA severaltimes.

The obtained double cross-linked HA sheet is insoluble in water and wasfound to pick up ten-folds of water to form a transparent gel. It alsoshows very good mechanical strength, which is an important feature fortissue engineering.

Solid State ¹³C NMR Analysis of HA Samples

The solid-state ¹³C NMR analysis of the hyaluroan and the two doublecross linked samples was carried at 50 MHz using an Advance 200spectrometer. The spectra obtained using a contact time of 1 ms in thestandard cross polarisation (CP) pulse sequence are shown in FIGS. 1-3.A spectrum of sample No. 3 containing the internal standard,tetrakis(trimethyl)silane (TKS, chemical shift of 3.2 ppm), was alsoobtained using a contact time of 5 ms (FIG. 4). The peak assignmentsreferenced to TKS are as follows.

Chemical shift, ppm C═O in carboxyl and acetyl 170-180 C═O in ester formodified samples 165-170 (shoulder) C₁  95-110 C₂-C₅ plus OCH and OCH₂in modifier 65-90 C₆ 60-65 C—N 53-60 CH₂ in modifier not bound to O20-40 CH₃ in acetyl 20-25

Sample 1 (FIG. 1) is pure hyaluronic acid without modification. Theactual formulations for sample 2 and sample 3 are shown in followingTable:

FIRST CROSSLINKING SECOND CROSSLINKING Reaction Reaction ReactionReaction feeding temperature time feeding temperature time ratio pH (°C.) (hours) ratio PH (° C.) (hours) Sample 2 3/1 10 RT 72 Sample 3 1/210 RT 2 hours 1/2 4 RT 2 hours

Sample 2 (FIG. 2): prepared according to the method of Example 3 butwithout the second crosslinking. The feeding ratio is the amount of HAto 1,2,7,8-diepoxyoctane. The formed film was cut into fine meshes forNMR analysis.

Sample 3 (FIGS. 3 and 4): prepared according to the method of Example 2to form a gel, which was milled to a fine powder for NMR analysis.

1. A process for the preparation of multiple cross-linked hyaluronicacid (HA), which process comprises covalently cross-linking HA via twoor more different functional groups, wherein said cross-linking iseffected by contacting HA with two or more chemical cross-linking agentsso as to form two or more different types of functional bonds between HAmolecules under one of an acidic or an alkaline condition withoutaltering such condition, and wherein said two or more different types offunctional bonds are selected from the group consisting of ether, ester,sulfone, amine, imino, and amide bonds.
 2. A process according to claim1 wherein the functional groups are selected from hydroxyl, carboxyl andamino.
 3. A process according to claim 1 wherein the cross-linking agentis selected from the group consisting of formaldehyde, gluteraldehyde,divinyl sulfone, a polyanhydride, a polyaldehyde, a polyhydric alcohol,epichlorohydrin, ethylene glycol diglycidylether, butanedioldiglycidylether, polyglycerol polyglycidylether, polyethylene glycoldiglycidylether, polypropylene glycol diglycidylether, and a bis-orpoly-epoxy cross-linker.
 4. A process according to claim 1 wherein anether bond is formed using a cross-linking agent selected from the groupconsisting of bis epoxides and poly epoxides under alkaline conditions.5. A process according to claim 1 wherein an ester bond is formed usinga cross-linking agent selected from the group consisting of bis epoxidesand poly epoxides under acidic conditions.
 6. A process according toclaim 4 wherein the cross-linking agent is selected from the groupconsisting of 1,2,3,4-diepoxybutane and 1,2,7,8-diepoxyoctane.
 7. Aprocess according to claim 1 wherein an ether bond is formed using agluteraldehyde cross-linking agent under acidic conditions.
 8. A processaccording to claim 1 wherein the cross-linking of each type offunctional group is effected sequentially.
 9. A process according toclaim 8 wherein HA is first cross-linked via the hydroxyl groups byformation of ether bonds and subsequently cross-linked via the carboxylgroups by formation of ester bonds.
 10. A process according to claim 1wherein the cross-linking of each type of functional group is effectedsimultaneously.
 11. A process according to claim 1 wherein said multiplecross-linked HA is cross-linked with two or more chemical cross-linkingagents so as to form two different types of functional bonds.
 12. Aprocess according to claim 11 which comprises: (a) cross-linking HA viaa first functional group and (b) subsequently cross-linking the productof (a) via a second functional group, wherein said first and secondfunctional groups represent different chemical entities.
 13. The processof claim 1, wherein said two or more different types of functional bondsare selected from the group consisting of ester and amine bonds.
 14. Aprocess for the preparation of multiple cross-linked hyaluronic acid(HA), which process comprises covalently cross-linking HA via two ormore different functional groups, wherein said cross-linking is affectedby contacting HA with two or more chemical cross-linking agents so as toform two or more different types of functional bonds between HAmolecules under one of an acidic or an alkaline condition withoutaltering such condition.